Financial Risk Management and Trading Activities - Corporate Financial Risk Management Activities - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended			12 Months Ended			3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		3 Months Ended		9 Months Ended
	Jun. 30, 2014 BOPD	Mar. 31, 2014 BOPD	Dec. 31, 2013 BOPD	Dec. 31, 2013 BOPD	Sep. 30, 2014 Interest Rate Swaps [Member]	Dec. 31, 2013 Interest Rate Swaps [Member]	Sep. 30, 2014 Sales and Other Operating Revenues [Member]	Sep. 30, 2013 Sales and Other Operating Revenues [Member]	Sep. 30, 2014 Sales and Other Operating Revenues [Member]	Sep. 30, 2013 Sales and Other Operating Revenues [Member]	Sep. 30, 2014 Interest Rate Swaps and Fixed-rate Debt [Member]	Sep. 30, 2013 Interest Rate Swaps and Fixed-rate Debt [Member]	Sep. 30, 2014 Interest Rate Swaps and Fixed-rate Debt [Member]	Sep. 30, 2013 Interest Rate Swaps and Fixed-rate Debt [Member]	Jun. 30, 2014 West Texas Intermediate [Member] BOPD	Sep. 30, 2014 Corporate Risk Management [Member] Closed & New Brent Crude Hedges [Member]	Sep. 30, 2014 Corporate Risk Management [Member] New Brent Crude Hedges [Member]	Sep. 30, 2013 Corporate Risk Management [Member] New Brent Crude Hedges [Member]	Sep. 30, 2014 Corporate Risk Management [Member] New Brent Crude Hedges [Member]	Sep. 30, 2013 Corporate Risk Management [Member] New Brent Crude Hedges [Member]
Derivative [Line Items]
Total number of barrels per day hedged based on crude oil fixed-price swap contracts			25,000	90,000											20,000
Total number of additional barrels per day hedged based on Brent crude oil fixed-price swap contracts	10,000	5,000
Average price per barrel	109.17	109.17		109.70											100.41
Realized gains from E&P hedging activities due to the increase in sales and other operating revenues							$ 27	$ 2	$ 24	$ 36
Realized gains from E&P hedging activities net of income taxes							17	1	15	23
After-tax deferred gains (losses) in Accumulated other comprehensive income (loss)																36
Gain (loss) on ineffectiveness reflected in earnings																	6	(17)	2	1
Derivative notional amount					1,300	865
Change in the carrying value of the hedged fixed-rate debt - increase/(decrease)											(10)	1	(5)	(27)
Change in the fair value of interest rate swaps - increase/(decrease)											$ (10)	$ 1	$ (5)	$ (27)